Separation of exhumation and post-intrusion cooling with thermochronology, Al-in-Hbl geobarometry, and numerical thermal modeling; An Example from Central Japan

熱年代学、Al-in-Hbl地質圧力計、数値熱モデルによる削剥と貫入後の冷却の分離; 中央日本の例

南 沙樹*; 末岡 茂   ; 福田 将眞   ; Malatesta, L.*; 河上 哲生*; 東野 文子*; 梶田 侑弥*; 田上 高広*

Minami, Saki*; Sueoka, Shigeru; Fukuda, Shoma; Malatesta, L.*; Kawakami, Tetsuo*; Higashino, Fumiko*; Kajita, Yuya*; Tagami, Takahiro*

The Tanigawa-dake area, in the southern end of the Northeast Japan arc, hosts such granites of late Miocene to Pliocene ages ranging from 6.0-5.5 Ma, ca 4.0 Ma to 3.3-3.2 Ma (zircon U-Pb). Previous studies also reported zircon (U-Th)/He dates (ZHe) of 3.3-1.4 Ma and apatite (U-Th-Sm)/He (AHe) dates of 2.8-1.0 Ma for these young granites and the Cretaceous granites. Exhumation rates of 0.3-1.7 mm/yr were estimated by AHe dates and assumption of constant geothermal gradients of 40-60 K/km. However, the AHe dates might reflect initial cooling phase of the young plutons as well as cooling derived from exhumation, potentially leading to an overestimation of exhumation rates in the Tanigawa-dake area. This study aims to constrain a more reliable exhumation history. We applied two methods for the youngest pluton (ca 3.3 Ma): (1) Al-in-Hbl geobarometry to estimate the emplacement depth and (2) 1D numerical simulation of geothermal structure based on heat advection-diffusion-production equation to explore the best cooling/exhumation histories consistent with the reported zircon U-Pb age, ZHe and AHe dates. As a result of Al-in-Hbl geobarometry, solidification pressures of 0.9-2.6 kbar were estimated. Emplacement depths derived from these pressures are 3.4-9.5 km. Exhumation rates were calculated to be 1.0-2.9 mm/yr for the youngest pluton, assuming an intrusive age of ca 3.3 Ma. In the 1D heat advection-diffusion-generation model, the best exhumation rates are ca 1.2 mm/yr and the best emplacement depth is ca 4.0 km. Comparing with the exhumation rate estimated from the AHe age of ca 1.0 Ma in the same pluton, the exhumation rates by geobarometry are equal to or higher than the rate by AHe date. Similarly, the modeled rate fits with the exhumation rate by AHe age. This indicates that the initial cooling was finished by the time of the AHe date for ca 3 Ma pluton, i.e., the previous geothermal structure in this area had relaxed to the current one.